Tumor associated genomic rearrangements can give raise to structurally altered products with aberrant biological properties. Chromosomal translocations frequently result in gene fusions that can behave as dominant oncogenes. One such family of translocations occurs in Ewing's sarcoma/primitive neuroectodermal tumor (ES/PNET), and juxtaposes the N-terminus of EWS to the C-terminus of 5 possible ETS proteins. Inhibition of EWS/FLI1 fusion attenuates growth of ES/PNET cells, implicating EWS/FLI1 in the genesis and maintenance of these tumors. Present data suggest that EWS/FLI1 fusion promotes anchorage independent growth and tumorigenesis by acting as aberrant transcription factors. While a cohort of EWS/FLI1 target genes have been identified, it appears that transcriptional modulation of a larger network of genes is necessary to cause these biological effects. This proposal aims to clarify how EWS/ETS genes promote oncogenesis by 1) identifying and cataloging a more complete set of genes whose expression is altered by the EWS/FLI1 fusion protein(s); 2) defining which of the aberrantly modulated target genes are the direct/immediate targets of EWS/FLI1 transcriptional modulation (as opposed to downstream members of a gene expression cascade); and 3) developing better model systems to determine which combinations of EWS/FLI1 target genes play causal roles in EWS/FLI1-driven process of oncogenesis.